A number of different electronic identification methods and devices have been developed and are presently being used. We are all familiar with the ubiquitous bar codes and magnetic strips that, together with their readers, are widely employed by businesses. An inherent limitation in bar code and magnetic strip identification devices is the effective range at which they can be reliably read, which distance is quite short. Magnetic strips, for example, generally require the reader to be in direct contact with the strip in order to detect and decode data. In the very few cases where a magnetic strip is read with a device other than a direct contact reader, the effective reading range is still only a few centimeters at best. Similarly, the effective range at which bar codes can be reliably read is typically no better than a few centimeters. Because the read range for bar codes and magnetic strip is so short, they are usually read one at a time and seldom does one bar code or magnetic strip interfere with another.
Another prior art identification device is the radio frequency identification (“RFID”) tag. When interrogated, RFID tags reflect or retransmit a radio frequency signal that returns an encoded identification number. Although these prior art RFID tags are based on a chip that has a longer reliable read range than magnetic strips or bar codes, they are expensive and not widely used. As a general rule, prior art RFID tags are read individually, leaving little opportunity for a coded response from one such tag to interfere with the coded response from another.
With the introduction of inexpensive identification tags based on surface acoustic wave (SAW) technology that can be read at a relatively long range, circumstances will arise where significant interference problems can occur from multiple tags returning coded responses. There is, of course, not a problem when an individual SAW identification tag is read, or even when a small group of SAW tags are read simultaneously.
Significant code collision problems occur when a large number of SAW tags are simultaneously interrogated and each tag simultaneously returns a coded response. Such is the case, for example, when a pallet of articles, each identified with a SAW identification tag, is interrogated and each tag returns a globally unique response code. Such a large number of coded response pulses makes it difficult for a SAW identification tag reader to accurately detect and decode each response and reliably identify each article on the pallet. Code collision problems as well as other inter-symbol interference problems caused by so many responses being transmitted at one time needs to be addressed before the full potential of SAW identification tags can be realized.
Accordingly, what is needed in the art is a system for focusing an interrogation pulse to within a defined space so that only SAW identification tags located within that space respond to an interrogation pulse, thus enabling a SAW identification tag reader to discriminate between coded responses.